This invention is a further improvement to applicant's earlier inventions in this field that are generally entitled "Air Ride Boat Hulls". All of these inventions require the introduction of pressurized gas into a recess in the underside of a boat hull to increase operating speeds and load carrying capabilities and improve stability. The improvements are the result of the pressurized gas cushion supporting some eighty to ninety percent of boat weight in most cases. The efficiency improvements at high speed are mainly the result of a reduction, normally by several orders of magnitude, of wetted area drag compared to a more conventional hull form.
The instant invention offers significant advances over applicant's earlier patents from the standpoint of use of multiple, primarily parallel and widely spaced, hulls. The effect of this is to change and improve vessel stability and resistance and performance characteristics, in many instances, compared to a more generic air cushion supported marine craft such as the Surface Effect Ship (SES). The SES has thin parallel sidehulls that are joined transversely fore and aft by full span flexible seals--the space thus formed is pressurized with air to provide a large lifting force. As such, it can be seen that the generic air cushion supported marine craft has a single rather wide rectangular footprint on the water's surface compared to the two or more widely spaced narrower and finer, preferably pointed bow shaped, footprints of the instant invention.
The widely spaced long and thin air cushion hulls, in combination, have a much reduced low or "hump" speed resistance characteristic than does a single wide air cushion hull. However, at very high speeds a large wide air cushion hull with its single cushion generally has less resistance. That is due in part to the added wetted area resistance of the necessary extra sidewalls on the inboard sides of the separated catamaran-like sidehulls of the instant invention. Some effort has been made in the instant invention to reduce that wetted area effect by use of new concept vertically oriented steps that are preferably vented by surface air supplied downward along the vertical steps. It is preferable to have the sidewalls taper inward going aft in way of the vertical steps to reduce the drag force due to the impingement of water spray on the steps.
The instant invention accomplishes the preceding, in its optimum configuration, while using essentially boat shaped sidehulls that, in their preferred embodiment, have a generally pointed bow and truncated stern shape as seen in a dynamic waterline plane where the water contacts the sidehull. At least one of the sidehull keels diverges outward aft of the bow and then become substantially parallel over most of its after length. The sidehulls are symmetrical in the preferred embodiment; however, asymmetrical sidehulls can be used. Use of fixed and/or movable seals can also be used in or proximal to the sidehull pressurized gas recesses.
The U.S. Navy funded some studies in the early eighties of a boat that utilized catamaran-like sidehulls with each sidehull a generic SES. As stated before, the generic SES is a pressurized air cushion vehicle that incorporates full span flexible seals fore and aft (bow and stern) between parallel sidehulls. In the Navy's concept, the twin generic SES sidehulls were separated by and connected to a cross deck or wetdeck structure. Not only were their sidehulls parallel, but each sidehull had sides that were thin and plate-like and ran completely up to the wetdeck. Outward extensions of the wetdeck actually formed the upper surface of the pressurized gas recesses. This can be seen from FIG. 2, page 302, from an article titled: "The Surface Effect Catamaran--Progress in Concept Assessment" by F. W. Wilson, et al, pages 301-311, Naval Engineer's Journal, May 1983, published by the American Society of Naval Engineers, Inc., 1452 Duke Street, Alexandria, Va. 22314. That article gives a detailed summary of the Navy's catamaran sidehull air cushion vehicle project. Comparison of the vehicle described in that article and the instant invention clearly shows the significant conceptual improvements and, in most cases, simplifications that the instant invention offers over the Navy's concept. For purposes of simplification, the Navy's project will be referred to as SECAT (their acronym made from Surface Effect CATamaran) in the remainder of this document.
SECAT's idea was that each sidehull's flexible bow seals would give to waves and they could pass through the sidehull recess without molestation unless they were of sufficient height to contact the recess wetdeck. While the SECAT's analytical and model studies were encouraging there appears to have been no follow-on efforts on actual full size or prototype vessels. SECAT also requires extra structural weight for the long thin cantilevered sidehull plates and would have very poor off-cushion performance. It would appear that concern for the aforementioned plus the very high, narrow, and high maintenance flexible seals in each sidehull recess with the related tremendous increase in wetted area resistance in high seas, due to the extra vertical movement of the flexible seals on the inboard sides of each sidehull and resulting increases in wetted area, compared to a generic single chamber SES, probably discouraged further efforts.
The instant invention's sidehulls, in their preferred embodiment, have substantially pointed bows that allow the use of shallow recesses since the pointed bows divide an oncoming wave and direct it to either side of a sidehull rather than allowing its full passage into a sidehull's recess. The instant invention's sidehulls have rather boat shaped pointed bows and truncated sterns, as seen in outline in a horizontal waterline cross section of a sidehull, in its preferred embodiment. This compares to the SECAT where each of SECAT's sidehulls have a rectangular footprint on the water surface as can be seen in FIG. 11, page 306, of the preceding referenced SECAT article. Further, the aforementioned new concept vertical steps in the sidehulls of the instant invention will greatly limit the increase of wetted area resistance on the outside or sea surface sides of the sidehulls when operating in heavy seas. Neither of the just mentioned advantages of the instant invention reduce its performance and will, indeed, provide for at least reasonably good conventional catamaran-like hull performance with the blower system off. That is because the relatively shallow gas cushions and optional recess seal members make for much more hull like undersurfaces than did the SECAT's twin air cushion design. That is because the SECAT's design used full, to the wetdeck, depth recesses with full depth flexible seals fore and aft. That design used parallel thin sideboards, that extended all the way to the wetdeck, to form each side of the catamaran sidehulls. It can easily be seen that the SECAT's design must have functioned much like a lilypad with the blowers off.
The use of seals to divide portions of the recess also offer advantages. They reduce the amount of recess surface wetting when operating in heavy seas. This is particularly so when the preferred inverted-V seal design is utilized since such a shape has its lowest point at the sidehull sides thereby directing waves away from recess vertical surfaces downstream of the inverted-V seals. The recess seals also provide fore and aft smaller recesses that can be pressurized differently to help trim the vessel. The differential pressurization is accomplished by use of pressure control valves that are directed by an on-board controller that senses hull orientation and pressures in the recesses.
The instant invention also offers a center bow disposed inward, and generally in part forward, of the sidehulls. This offers significant additional advantages. The first is in appearance where a rather yacht-like pointed bow, as seen from topside or in profile, can be realized if the center bow extends forwardly. The second is in providing good ride qualities in rough seas in that the center bow helps to prevent bow pitch down of the boat in heavy seas. The third is that the center bow helps prevent wetdeck slamming when running in very heavy seas as it tends to lift the forward part of the boat as a large wave is encountered and then directs the wave away from the wetdeck.
Lake, U.S. Pat. No. 1,307,135, uses dual gas cushion floats with the floats supplied with exhaust gas from an engine. Lake's main intent is to make a device to aid in performance of a seaplane, or hydroaeroplane as he calls it, when the seaplane is either waterborne or airborne as can be realized upon examination of lines 96-106, page 2, of his patent which states: "Furthermore, it will be appreciated, that when the hydroaeroplane is in flight through the air, a corresponding entraining action will ensue between the aquafoils and supporting pressures, supplemented of course by the exhaust pressures, and the resulting diminution in frictional resistance, in this instance, will be measurable because of the relatively lighter density of the elastic fluid which is thus introduced between the aquafoils and the air pressure.". This is further shown by lines 22-24, page 2, of Lake which states: The float is shown as supporting a fuselage 8 by struts 9, and aerofoils representing a supporting unit are indicated by 10.". The supporting unit 10 is actually the wing of an aircraft. While his invention can also be applied to marine surface craft, it is obvious that much of the inventive thrust was directed at the seaplane or hydroaeroplane application which led to limited use as a marine surface craft. Applicant's invention has a distinctive racy yacht-like boat look with a pointed main bow and catamaran-like sidehulls that are flush with the boat sides in its preferred embodiment.
Some particular points regarding Lake's invention compared to applicant's instant invention are now noted. The gas cushion in each of Lake's floats are rectangular as seen in a calm sea surface waterline, when the float is pressurized with gas, and has parallel sides 2 and 3 over the length of the gas cushion. It is a preferred arrangement of applicant's invention that gas cushions be narrower, normally at least twenty-five percent, forward than aft which allows for a fine entry bow with little increase in wetted area on the lower surfaces of the bow at the bow portions of the recess. Further, also in the preferred embodiment of applicant's instant invention, a gas cushion bounding side of a catamaran-like sidehull would expand outwardly from its forward portions and then become more parallel over its aft portions. This allows for a finer entry forward coupled with most gas cushion bottom water contacting surface area and best streamlining aft. Further, when operating with the gas cushion pressurized and at high speeds, planing speeds for example, in calm seas the pointed bow portions of Lake's invention would be well clear of the water and the forward water contacting surface would be the transverse aft section of his forward bow 4 only. Applicant's preferred embodiment has a narrower bow shaped portion in water contact at high and low speeds. For purposes of applicant's instant invention, high speeds are defined as speeds of more than 25 knots.
Lake offers a series of transverse aquafoils 6 separated by spaces 7 across the lower portions of his gas cushions. Since there aquafoils must also serve as aerofoils when his craft is airborne and must further help direct exhaust gas flow out from the underside of the float they are separated and airfoil shaped. Therefore, he does not have separate recesses in a float whereas applicant's instant invention, in its preferred embodiment, utilizes separate recesses in each catamaran-like sidehull which, prefereably, can be supplied with gas at different pressures to aid in control of boat motions. Importantly also, Lake's aquafoils 6 are planar over their entire width, as seen in vertical transverse planes of his vehicle, which makes for a hard jarring ride in rough seas as compared to applicant's preferred embodiment angled recess seal surfaces. There are numerous other innovations in applicant's instant invention that offer dramatic improvements over both SECAT and Lake as are presented hereinafter.
Applicant has discussed all of the other related art in his earlier single hull Air Ride boat hull patent applications. The instant invention offers advancements over applicant's earlier inventions as well as over the prior art. These advancements are discussed in some detail in the following sections.